Techniques for generating multiple audio signals from a set of fewer audio signals have been developed for many years and are used in a variety of upmixing devices such as the Dolby Pro Logic II decoder described in Gundry, “A New Active Matrix Decoder for Surround Sound,” 19th AES Conference, May 2001. The perceived performance of the upmixing devices can generally be improved by decorrelation because at least some degree of decorrelation in the upmixed signals generally increases the perceived width of the aural image achieved by playback of the upmixed signals. Decorrelation can be obtained in a variety of known ways including simple delays and more complicated all-pass lattice filters.
Many conventional upmixing devices use one or more matrix structures to derive a number M output audio signals from a number N input audio signals, where N is less than M. Some devices use active or variable matrix structures that are adapted in response to control signals derived from the input audio signals. When decorrelation is used, an active matrix structure is sometimes divided into two stages. The first stage derives 2M intermediate signals from the N input audio signals and the second stage derives the M output audio signals from the 2M intermediate signals. A decorrelation technique is applied to half of the 2M intermediate signals. The second stage generates output audio signals with varying degrees of correlation by mixing amounts of non-decorrelated and decorrelated signals that are adapted in response to the control signals.
The choice of decorrelation technique can have a profound effect on the performance of an upmixing device. The inventors have determined that the performance of an upmixing device can be improved significantly if the decorrelation technique can satisfy three requirements simultaneously: provide a decorrelated signal that does not sound significantly different from the non-decorrelated signal, provide a sufficient amount of decorrelation to ensure the decorrelated signal sounds discrete or distinct with respect to the non-decorrelated signal, and allow mixing of the decorrelated signal and the non-decorrelated signal without generating audible artifacts. An additional advantage of such a technique is that the upmixed signals can be downmixed to a fewer number of input audio signals without generating objectionable artifacts.